I don't understand why you think this is true when I specifically described a label that is applied to "an individual section" of the page.
To be more explicit: Today, each post on the Reddit front page appears in its own container element. If an 18+ post's container element could have some kind of "adult-content" attribute set then some web browsers could render the whole front page normally with the exception of that individual element. Perhaps they could black it out, collapse it, display it with a pixelated overlay and a "Request Access" button, whatever the browser supports and the device administrator prefers.
> The solution is obvious: you show it on the front page if the user is 18+. However, your proposal deliberately forbids this and says the front page must be the same for everyone.
Why do you think that my proposal requires that the front page must be the same for everyone? Providing different views to different users is the entire point of every child safety law and rating scheme I've ever seen. I'm merely saying that my preferred approach would be to mandate that potentially objectionable content is semantically annotated in the HTML somehow, so that each user's browser can apply whatever view restrictions (if any) the device's owner wishes. I think this is better than mandating that web browsers send personal information to every web site they visit and also mandating that sites pre-filter the content they send back.